Multipath Propagation

 Multipath propagation and fading are two fundamental concepts in wireless communications that significantly impact the performance and design of mobile and wireless networks. Both are naturally occurring phenomena caused by the environment in which the signal travels from the transmitter to the receiver.

Multipath Propagation

Definition: Multipath propagation occurs when wireless signals travel from the transmitter to the receiver via different paths. These paths can involve bouncing off buildings, hills, or other objects, traveling through different mediums, or diffracting around obstacles.

Effects:

• Constructive and Destructive Interference: As the multiple copies of the signal arrive at the receiver at slightly different times, they can interfere constructively (amplifying the signal) or destructively (reducing the signal strength).
• Inter-Symbol Interference (ISI): In digital communications, different timing of multipath signals can lead to overlap of multiple symbols sent over the same channel, which can distort the received signal and make it difficult to decode correctly.
• Rapid Signal Fluctuations: The amplitude and phase of the received signal can change rapidly over short distances or time intervals.

Fading

Definition: Fading refers to the variation or attenuation of a signal's strength when it reaches the receiver, caused by various environmental factors including multipath propagation and the movement of objects.

Types of Fading:

1. Fast Fading: Occurs due to rapid fluctuations in amplitude and phase of the received signal caused by interference among the multiple different paths in multipath propagation, especially when the relative motion between the transmitter and receiver changes the path lengths quickly.
2. Slow Fading: Happens due to changes in the profile of the paths over longer periods or larger geographical scales, such as the physical movement of obstacles blocking the line-of-sight path or changes in the surrounding environment.
3. Rayleigh Fading: This type of fading assumes there is no dominant propagation along a line of sight between the transmitter and receiver, typical in urban environments with many obstacles.
4. Rician Fading: Occurs when one of the multipath components is dominant, typically the line-of-sight path, combined with numerous other weaker paths.

How Multipath Fading is Addressed

Equalization: To combat ISI caused by multipath delays, equalizers are used at the receiver to correct the distortion in the signal caused by the time delay differences of different paths.

Diversity Techniques: Employing multiple antennas at the transmitter or receiver (spatial diversity), using different frequencies (frequency diversity), or different times (time diversity) can mitigate the effects of fading by providing multiple independent paths for the same information.

Spread Spectrum Techniques: Technologies such as CDMA spread the transmitted signal across a wider frequency band. This makes the signal more resistant to narrowband interference and the effects of fading.

OFDM (Orthogonal Frequency-Division Multiplexing): Used in modern systems like LTE and WiFi, OFDM divides a wideband channel into many narrowly spaced sub-channels or subcarriers, which are orthogonal to one another. This arrangement allows the system to handle severe multipath conditions more effectively, as each subcarrier can be individually adapted to mitigate ISI and fading.

Summary

Multipath propagation and fading are key challenges in wireless communication that can degrade signal quality and reliability. Understanding these phenomena is crucial for designing robust wireless systems that can deliver high-quality service even in challenging environments. By using advanced technologies like MIMO, OFDM, and diversity techniques, modern wireless systems can overcome the detrimental effects of these phenomena to a significant extent, thereby improving performance and user experience 

Mobile Cellular Environment

 The mobile cellular environment involves a complex interaction of radio signals, mobile devices, and network infrastructure designed to support seamless communication over a broad geographic area. Understanding the basics of this environment is crucial for developing, deploying, and managing cellular networks. Here are some key concepts and components that define the mobile cellular environment:

1. Cellular Network Structure

Cells: The entire network is divided into small geographic areas called "cells," each served by at least one fixed-location transceiver known as a base station. These cells are typically hexagonal, square, or circular in shape. The idea is to cover the region thoroughly but without excessive overlap.

Base Stations (BS): Each cell has a base station that manages the radio communications with all mobile devices (cell phones, tablets, etc.) within its cell. The base station connects to a wider telephone network and the Internet.

Mobile Switching Center (MSC): This is the central component of a cellular network that handles call routing, mobility management, and additional services such as billing and handovers for mobile users moving between cells.

2. Frequency Reuse

To efficiently use the available spectrum, cells are assigned a group of frequencies which are reused in other cells that are sufficiently distant to avoid interference. This concept, known as "frequency reuse," allows for extensive coverage and capacity across a network without requiring additional spectrum.

3. Handover

As mobile users move from one cell to another, their ongoing calls or data sessions need to be transferred seamlessly from the current base station to the next. This process is known as "handover" or "handoff." It ensures that there is no interruption as users move around in a cellular network.

4. Modulation and Access Techniques

FDMA (Frequency Division Multiple Access): Each call uses a different frequency.

TDMA (Time Division Multiple Access): The channel is divided into different time slots.

CDMA (Code Division Multiple Access): Every channel uses the full available spectrum, and each call is differentiated by a unique code.

OFDMA (Orthogonal Frequency-Division Multiple Access): Used in LTE and 5G, it is a variant of FDMA that uses multiple subcarrier frequencies to enhance spectral efficiency, reduce interference, and increase resilience to multipath fading.

5. Path Loss, Fading, and Multipath

Path Loss: The attenuation of signal strength with distance due to the spreading of radio waves and absorption or reflection by obstacles like buildings or trees.

Fading: Variability of signal strength caused by various environmental factors, including multipath reception, where the signals arrive at the receiver through multiple paths caused by reflections.

Multipath: Occurs when transmitted signals reflect off surfaces such as buildings or mountains before they reach the receiver. This can result in signal fading, delay spread, and interference.

6. MIMO (Multiple Input Multiple Output)

A technology used to multiply the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO is a key part of modern wireless communication standards like LTE and 5G.

7. Environmental Challenges

In a mobile cellular environment, various physical and electronic factors such as terrain, buildings, weather conditions, and other electromagnetic signals can affect signal propagation and quality.

8. Emerging Technologies and 5G

Newer cellular technologies such as 5G aim to significantly improve speed, reduce latency, increase capacity, and allow for massive device connectivity. 5G networks utilize higher-frequency bands with greater capacity and shorter range, requiring a denser network of cells known as "small cells."

Summary

The mobile cellular environment is a dynamic and complex field involving various technologies and concepts. From the basic structure of cells and the application of frequency reuse principles to the advanced technologies like MIMO and 5G, the cellular network environment continuously evolves to meet the growing demands for faster data rates and more reliable service in an increasingly connected world.

Wireless Technology

UNIT I INTRODUCTION TO WIRELESS COMMUNICATION SYSTEM

An overview of wireless communication and future vision. Wireless communication system and standards: satellite communication system, GPS, paging system, cordless phone, wireless local loop, RFID.

Unit II: The cellular fundamentals: 

cellular communication and frequency reuse, general architecture of a cellular system, channel assignment strategies, hand-off in a cellular system. Interference and cellular system capacity: co-channel interference and adjacent channel interference, power control, evolution of mobile cellular communication: different generations of mobile cellular communication (1G, 2G. 2.5G, 3G and beyond), typical cellular standards (AMPS, GSM, GPRS, WCDMA, LTE, concept of LTE-advanced), 4G features and challenges, 5G vision. 

Unit III: Signal propagation in mobile communication : 

mobile cellular environment, multipath propagation and fading, free space propagation model, propagation path loss, outdoor propagation models (Okumura model & Hata model), indoor propagation models, power delay profile, channel parameters (delay spread, doppler spread, coherence bandwidth, coherence time, LCR and ADF). (8) 

Unit IV: Wireless Communication Networks : Wireless Personal Area Networks (Bluetooth, UWB and ZigBee), Wireless Local Area Networks (IEEE 802.11, network architecture, medium access methods, WLAN standards), Wireless Metropolitan Area Networks (WiMAX), Ad-hoc Wireless Networks. 

Unit V: Multiple access schemes: duplexing schemes, FDMA, TDMA, SDMA, spread spectrum technique and CDMA, OFDMA, ALOHA and CSMA.

GSM Specifications

The GSM specifications define the interaction between system components through well-defined interfaces and protocols. Figure 3-11 shows the interfaces between the GSM functional entities. Table 3-3 lists the GSM interfaces.

Figure 3-12 shows the protocol architecture used for the exchange of signaling messages on each interface. The protocols are layered according to the OSI Reference Model. It consists of the Physical Layer, Data Link Layer, and Layer 3. This Layer 3 is not the same as defined in OSI Layer 3. In GSM, the Layer 3 functions include call, mobility, and radio resource management. In the OSI model, these functions are provided by the higher layers. GSM reuses a few established protocols such as CCS7 MTP, TCAP, SCCP, ISUP, and ISDN LAPD protocols. The MAP and BSSAP are new protocols to support GSM specific needs.

Air interface

The air interface between the MS and the BTS is called Um. The GSM air interface is based on time division multiple access (TDMA) with frequency division duplex (FDD). TDMA allows multiple users to share a common RF channel on a time-sharing basis, while FDD enables different frequencies to be used in uplink (MS to BTS) and downlink (BTS to MS) directions. Most of the implementations use a frequency band of 900 MHz. The other derivative of GSM is called Digital cellular system 1800 (DCM1800).

Figure 3-11 GSM interfaces.

TABLE 3-3 GSM Interfaces

Interface	Description
Um
Abis
A
B
C
D
E
F
G
H

It uses a frequency band of 1800 MHz. Table 3-4 lists the GSM frequency bands.

The used frequency band is divided into 200-kHz carriers or RF channels in both the uplink and downlink direction. Each RF channel is then further subdivided into eight different timeslots, i.e., 0 to 7, by TDMA techniques. A set of these eight timeslots is referred as a TDMA frame. Each frame lasts 4.615 ms. The physical channels are further mapped to various logical channels carrying user traffic and control information between the MS and the BTS. Table 3-5 describes the logical channels and their usages.

The following section describes the Um interface protocols used at the MS and the BTS side.

Physical layer. Layer 1, which is a radio interface, provides the functionality required to transfer the bit streams over the physical channels on the radio medium. The services provided by this layer to those above include:

■ Channel mapping (logical to physical)

■ Channel coding and ciphering

■ Digital modulation

■ Frequency hopping

■ Timing advance and power control

Data link layer. Signaling Layer 2 is based on the LAPDm protocol, which is a variation of the ISDN LAP-D protocol. The main task of LAPDm is to provide a reliable signaling link between the network and the mobile station. The LAP-D protocol has been modified to adapt in the mobile environment. For example, LAPDm uses no flags for frame delimitation. The Physical Layer itself does the frame delimitation. This way, scarce radio resources are not spent on flag bits the bit.

Figure 3-12 GSM protocol stack.

TABLE 3-4 GSM Frequency Bands

System	Direction	Frequency band (MHz)
GSM 900	Uplink	890-815
	Downlink	935-960
GSM/DCS1800	Uplink	1710-1785
	Downlink	1805-1880

Network layer. Signaling Layer 3 takes care of signaling procedures between an MS and the network. It consists of three sublayers with distinct signaling procedures.

■ Radio resource management (RR)

■ Mobility management (MM)

■ Connection management (CM)

Radio resource management. Radio resource management (RR) comprises procedures required to establish, maintain, and release the dedicated radio connections. The RR sublayer functions include:

■ Channel assignment and release

■ Ciphering

■ Modification of channel modes, e.g., voice and data

■ Handover between cells

■ Frequency redefinition to enable frequency hopping

■ MS measurement reports

■ Power control and timing advance

■ Paging

■ Radio channel access

The mobile station always initiates an RR session. For example, the RR procedures are invoked to establish an RR session in response to a paging message or to establish an outgoing call. As shown in Figure 3-13, the RR messages are transferred to BSC transparently, through the BTS. Table 3-6 lists RR messages.

Mobility management. The mobility management (MM) sublayer handles functions and procedures related to mobility of the mobile user. This includes procedures for:

■ Authentication

■ Location registration and periodic updating

TABLE 3-5 Logical Traffic and Control Channels

TABLE 3-5 Logical Traffic and Control Channels

	Traffic channels (TCH)
	TCH/F carries subscriber information (speech/data) at a rate of 22.8 Kbps with a speech coding at around 13 Kbps.
	TCH/F carries subscriber information at a rate of 11.4 Kbps with a speech coding at around 7 Kbps.
	Broadcast control channels (BCH)
	This channel is broadcast by the BTS and carries information for the frequency correction of the MS. It is used in downlink direction only.
	This channel is broadcast by the BTS and carries information for frame synchronization of the MS. In addition it also carries the base station identity code (BSIC). It is used in downlink direction only.
	This channel carries broadcast information related to the BTS and the network. The information includes configuration details of common control channels (CCH) described below.
	It is used in downlink direction only.
	Common control channels (CCH)
	This is used to page an MS. It is used in downlink direction only.
	The MS uses this channel to request the allocation of a SDCCH. It is used in uplink direction only.
	The BTS allocates a SDCCH or TCH in response to the allocation request by the MS using this channel. It is used in downlink direction only.
	Dedicated control channels (DCH)
	This channel is used for carrying signaling information between the BTS and a MS before allocation of a TCH. For example, SDCCH is used for carrying signaling messages related to update location and call establishment. This is a bidirectional channel.
	This channel is always used in conjunction with a TCH or a SDCCH. The MS and the BTS use it to maintain an SDCCH or a TCH. In the uplink, the MS sends measurement reports to the BTS using this channel. In the downlink, the BTS transmits information to keep the mobile updated on recent changes in system configuration.
	This channel is always associated to a TCH and is used to transfer signaling messages when a mobile is already involved in a call.

 

Figure 3-13 Air interface signaling protocols.

■ Security

■ TMSI reallocation

■ IMSI detach/attach

As shown in Figure 3-13, the CM layer from the transmitting side uses the MM layer to establish RR connection and then transfers messages transparently across to the receiving side, that is MSC. Table 3-6 lists MM messages.

Connection management. The connection management (CM ) sublayer contains the functions and procedures for call control. This includes procedures to establish, release, and access services and facilities. The CM consists of three sublayers, namely, call control (CC), supplementary services (SS), and short message services (SMS).

The call control sublayer provides procedures for ISDN call control. These procedures are based on ISDN call control procedures defined in the ITU-T Q.931 specification. However, the minor modifications are done to adopt these to mobile environment.

The supplementary service sublayer provides the procedures to support non-call-related supplementary services such as call forwarding and call waiting.

TABLE 3-6 Layer 3 Messages

RR messages	MM messages	CM messages
Channel establishment messages	Registration messages	Call establishment messages
ADDitional ASSignment	IMSI DETach	ALERTing
	INDication
IMMediate ASSignment	LOCation UPDating	CALL CONFirmed
	ACCept
IMMediate ASSignment	LOCation UPDating	CALL PROCeeding
EXTended	REJect
IMMediate ASSignment	LOCation UPDating	CONnect
REJect	REQuest
Paging messages	Connection	CONnect
	management messages	ACKnowledge
PAGing REQuest Type 1	CM SERVice ACCept	SETUP
PAGing REQuest Type 2	CM SERVice REJect	EMERGency SETUP
PAGing REQuest Type 3	CM SERVice REQuest	PROGRESS
PAGing ReSPonse	CM SERVice ABOrt	Call phase messages
Handover messages	CM REeStablishment	MODify
	REQuest
ASSignment CoMmanD	Security messages	MODify REJect
ASSignment COMplete	AUTHentication REJect	MODify COMPlete
ASSignment FAILure	AUTHentication	USER INFOrmation
HANDover ACCess	REQuest
HANDover CoManD	AUTHentication	HOLD
	ReSPonse
HANDover COMplete	IDENTity REQuest	HOLD REJect
HANDover FAILure	IDENTity ReSPonse	HOLD ACKnowledge
PHYsical INFOrmation	TMSI REALlocation	RETRIEVE
	COMmand
Ciphering messages	TMSI REALlocation	RETRIEVEREJect
	CoMPlete
CIPHering MODe CoMmanD	Other messages	RETRIEVE
		ACKnowledge
CIPHering MODe COMplete	MM STATUS	Call clearing messages
Channel release messages	ABORT	DISConnect
CHANnel RELease		RELease
PARTial RELease		RELease COMplete
PARTial RELease COMplete		Other messages
System information messages		CONGESTion
		CONTROL
SYStem INFOrmation Type1		STATUS
SYStem INFOrmation Type2		STATUS ENQuiry
SYStem INFOrmation Type3		NOTIFY
SYStem INFOrmation Type4		START DTMF
SYStem INFOrmation Type5		STOP DTMF
SYStem INFOrmation Type6		START DTMF
SYStem INFOrmation Type7		ACKnowledge
SYStem INFOrmation Type8
SYStem INFOrmation		STOP DTMF
Type 2bis		ACKnowledge
SYStem INFOrmation		START DTMF REJect
Type 5bis

TABLE 3-6 Layer 3 Messages

RR messages	MM messages	CM messages
Other messages
CHANnel REQuest
CHANnel MODe MODify
CHANnel MODe MODify
ACKnowledgment
CLASSmark ENQuiry
CLASSmark CHANGE
FREQuency
REDEFinition
RR Status
MEASurement REPort

The short message service sublayer provides the procedures to support the short message transfer between the MS and the network. Table 3-6 lists CM messages.